In manufacturing processes, multiple structural components are often manufactured separately and then assembled together. For example, tens or even hundreds (or more) of structural components may be assembled together to form structures in aerospace or aviation applications, such as in the manufacture of aircraft wings. Gaps between such structural components may result from manufacturing tolerances of the components and/or from unique challenges associated with certain materials. For example, in making composite parts, geometric variations in final parts may result from variations in fiber diameter and/or variations in resin volume, which may accumulate via a plurality of layers of material that are laid up to form the composite part.
In the assembly of a wing skin panel to ribs to form an aircraft wing-box, for example, some ribs may be in contact with the skin panel, while other ribs are spaced from it, due to such variations in the as-built ribs. FIG. 1 shows an example of a wing box structure for an aircraft wing 10. Generally, aircraft wing 10 includes a ladder-like structure formed by a plurality of ribs 12 spaced apart between one or more longer spars 14. Ribs 12 generally define the overall shape of aircraft wing 10, with a skin panel 16 (partially shown in dashed line, also referred to as a wing skin) being attached to ribs 12, conforming to the shape of the ribs 12. Because engagement between the skin panel 16 and ribs 12 is important to the structural strength of the wing-box, any gaps between them are generally filled by applying a liquid or solid shim between ribs 12 and skin panel 16, where needed (e.g., in the positions of the gaps). Placement of these shims is a generally time-consuming and expensive process. In some cases, composite structures may have to be assembled and disassembled several times to measure the shim gaps and drill and clean holes.
In some cases, material may be removed from the structural components and/or from the shims, to ensure proper engagement between the respective structural components and/or between a structural component and a shim. Cutting tools (e.g., milling machines with various end mill cutting tools) may be used to remove said material. For example, end mills are generally used for ruled milling of parts (which may also be referred to as plane milling, and generally results in cut areas with flat surfaces), and ball nose mills are generally used to cut curved or contoured areas in a process referred to as hemstitch milling. FIG. 2 illustrates an example of an end mill 18, which includes a shank 20 and a cutting portion 22. Cutting portion 22 is positioned adjacent a distal end region 24 of end mill 18, while a proximal end region 25 is defined by shank 20. Shank 20 is generally cylindrical and may be used to hold and locate end mill 18 in a milling machine. Cutting portion 22 includes one or more helical teeth 26, with one or more flutes 28 formed between helical teeth 26. Helical teeth 26 form a blade, or cutting surface, for removing material during cutting, while the removed material is moved up flute 28 during rotation of end mill 18, thereby clearing the material from the cutting surface. Helical teeth 26 are generally located on an end face 27 of end mill 18 (within distal end region 24), as well as on a periphery 29 of cutting portion 22 of end mill 18.
In use, and as shown in FIG. 3, end mill 18 generally is rotated about a longitudinal axis 30 of shank 20 (e.g., rotated as indicated by arrow 32), with distal end region 24 generally being vertically arranged (e.g., substantially normal to) the surface of a workpiece 34 being cut. When end mill 18 is rotated with respect to workpiece 34 in ruled milling applications, such an arrangement results in material being removed from workpiece 34, such as cut area 36 shown in FIG. 3.
FIG. 4 illustrates an example of a ball nose end mill 38, which may also be referred to as a ball end mill 38. Similar to end mill 18, ball nose end mill 38 includes shank 20 and cutting portion 22, with cutting portion 22 including one or more helical teeth 26 and one or more flutes 28 positioned between helical teeth 26. While the profile of end mill 18 is generally substantially flat at distal end region 24, ball nose end mill 38 generally is rounded, or semispherical at distal end region 24. In use, ball nose end mill 38 is passed across workpiece 34 multiple times and such hemstitch milling operations result in scallops 40, or ridges, formed in cut area 36 for each tool pass, based on the diameter of ball nose end mill 38.
Machining time is increased for each tool pass performed by the milling machine, and each time the cutting tool is changed. For example, in manufacturing processes where every second is significant, it may take a half an hour to change the tools between an end mill and a ball nose end mill, thereby increasing costs. While the time required for such tool changes is generally trivial in many manufacturing processes, it can be a significant expense in certain manufacturing scenarios, such as in assembling aircraft wings. Previous attempts at reducing machining time have involved tilting end mills at an angle with respect to the workpiece and attempting to use the end mill to perform the hemstitch operations, however, in doing so, just the tip of the end mill is able to be used in the cutting, which slows the speed at which the cutting is performed and results in uneven and excessive wear on the tools.